Vital train control systems rely on precise train-location determination, which is performed using an onboard track map in conjunction with GPS. During the course of operations, a train will often leave “controlled territory” and enter an industrial spur or branch track that is not owned or controlled by the operating railroad. Although all railroad-controlled track is mapped (and appears in the track map), industrial track in the vicinity of the railroad-controlled track is not necessarily mapped. This is due to cost considerations and the fact that the operating railroad does not own or control the industrial track.
Although the unmapped industrial track will not, of course, appear in the track map, the train control system must nevertheless support train operations where the train exits controlled territory, operates within the proximity of the point of exit, (e.g., on an industrial spur or branch track), and returns to controlled territory where all functions and features are enforced in a vital manner.